We've had this experience before when attempting to modify the configuration file. But because the user had no permission to write to "Program Files" directory this was not recommended. You cannot assume the user will always be administrator when using your app!

If a user doens't have priviledge to write to the program files directory then you use a manifest, the manifest will allow to the user to read tge app.config file, you need to do more investigation, or you are a very bad programmer.